This invention relates to a device for providing a visual indication of the heat or infrared radiation received from the sun. In particular, but not exclusively, the invention relates to a sun tan factor meter for providing a quantitative measure of the sun tanning power of the sun at any given time.
Electromagnetic radiation emitted by the sun covers a wide range of the spectrum. However a spectrum analysis of the emitted electromagnetic radiation shows that, on a clear day with little or no filtering cloud cover, there is a precise relationship between the proportion of ultra violet (U.V.) radiation emitted and the proportion of infra red (I.R.) radiation emitted. Thus a measurement of the temperature of a body heated by the sun (i.e. heated almost entirely by the I.R. radiation of the sun) will give a proportionate measurement of the U.V. radiation emitted by the sun on that body. Since the "burning" or sun tanning of the skin is caused, on the whole, by U.V. radiation emitted by the sun, it will be appreciated that measurement of the heat radiation from the sun received by a body can be used to provide an accurate measurement of the strength of the u.v. radiation from the sun.
It is known to provide complicated and expensive devices which electronically sense the temperature of solar radiation. It is also know to provide devices employing liquid crystal compositions which display temperatures over relatively narrow ranges--e.g. from 35.degree. C. to 40.degree. C. for a fever scope or from 10.degree. C. to 40.degree. C. for a room thermometer. Finally it is also known from U.S. Pat. No. 3,829,980 to provide a lid of a container for spraying sun tanning lotion with a device for assisting the user of the lotion. In this latter case the device is a gnomon of a sun dial for converting the lid into a sun dial so that the length of time a user is exposed to the sun can be measured.
None of the known devices provide a relaitively cheap device for providing an indiction of the strength of the heat radiation from the sun--e.g. to assess the sun-tanning power of the sun.